Lithium secondary batteries as typical batteries using a nonaqueous electrolyte have been used widely as power sources for such portable devices as a mobile phone and a notebook personal computer because of their high energy density. Since lithium secondary batteries use chemically highly active lithium ions and an electrolyte (nonaqueous electrolyte) containing a flammable organic solvent, various portions of the batteries are provided with safety mechanisms for preventing ignition and smoking under abnormal conditions. The capacities of lithium secondary batteries are likely to increase further as the performance of the portable devices becomes higher. Therefore, it is important to further ensure both safety and reliability.
In the current lithium secondary battery, for example, a polyolefin microporous film with a thickness of about 20 to 30 μm is used as a separator that is interposed between a positive electrode and a negative electrode. However, the separator that is used generally at present is likely to shrink when the temperature inside the battery becomes extremely high, which may lead to a short circuit. For this reason, studies to improve separators have been conducted to further improve the safety and reliability of lithium secondary batteries.
For example, Patent documents 1 to 4 each propose to use a separator including filler particles having excellent heat resistance and a resin component for ensuring a shutdown function for forming an electrochemical element such as a lithium secondary battery.